Useful resolution - any benefit to more megapixels?
The closest comfortable viewing distance for a print is about equal to its diagonal, I believe. Held normally at that distance it subtends a little under 50 degrees horizontally at the eye. Someone with good vision has an angular resolution of about 1 arc minute (any details separated by less than that are indistinguishable). Allowing one pixel per arc minute and the usual 3:2 width to height ratio, that makes a bit less than 6 MP in total. So for normal purposes (excluding certain specialised applications) what is the point of 15, 18, 20 MP? I'm not saying that there isn't any benefit, but I wonder whether more could be gained for the most common applications by applying the technological improvements that have gone into these dense sensors to something of lower resolution, and getting better sensitivity, dynamic range and colour (and smaller files). I guess that this has been discussed plenty of times before, but I'm curious as to where the tradeoffs are, especially as some of you have experience with this range of resolutions.
Another thing that occurs to me is that diffraction limiting kicks in at wider apertures with denser sensors. The diffraction limited linear resolution (smaller is better) is proportional to the focal length and inversely proportional to the aperture diameter (i.e. it is proportional to the F-number, which is another application of that useful quantity). So in order to take full advantage of a denser sensor you need a wider relative aperture. This would mean that applications that require a large depth of field, and therefore a small aperture, like landscape and high magnification (macro) benefit the least from very high sensor resolution, even if essentially perfect optics were available. I'm not saying that higher resolution makes things worse in this respect in the final output, but beyond a certain point it can't make things any better, and there might be other drawbacks. Even with a 10 MP APS-C sensor diffraction starts to have an effect at not a lot narrower than F8.
Will
Re: Useful resolution - any benefit to more megapixels?
I tend to agree with you. I've heard arguments about print quality and how you should multiply the size, say 11x14 by 300 dpi to get the megapixel count you need. I've just printed some 11x14 B&W from my 10mp Nikon D80 -- at Costco, no less -- and they are stunning even though the math says they're under-powered in the megapixel count, even if you stare at them up close. And I know I can do better with a higher quality printing service.
OTOH, I keep reading articles claiming that at least for B&W film, digital still doesn't produce the same sharpness/resolution, so I wonder if that's part of the megapixel drive, at least for FF/PRO.
Re: Useful resolution - any benefit to more megapixels?
Quote:
Originally Posted by
eNo
I tend to agree with you. I've heard arguments about print quality and how you should multiply the size, say 11x14 by 300 dpi to get the megapixel count you need.
Personally, I think that the people who insist on 300 dpi being a "minimum" requirement should do a little maths of their own.
Lets go metric for the sake of ease of discussion; there's more-or-less 25 millimeters in an inch, so 300 dpi would equate to 12 pixels in one millimeter, squared (because we're talking a 2 dimensional image) ...
... which equates to potentially 144 tone changes in a single square millimeter.
I don't know about anyone else, but my eyes just couldn't resolve 144 tone changes in a single millimeter at ANY viewing distance, and probably not even with a magnifying glass. So to my mind this 300 dpi thing is just silly.
Just my 10c worth!
Re: Useful resolution - any benefit to more megapixels?
Quote:
Originally Posted by
will_c
So for normal purposes (excluding certain specialised applications) what is the point of 15, 18, 20 MP?
FWIW one of the reasons I like lots of pixels is because I like photographing things that are too small to see clearly with the naked eye. Springtails mostly but there's plenty other small beasties that fascinate me.
Does using your camera as an expensive magnifying glass count as a specialised application? ;) I'm often shooting at (well and truly) diffraction limited apertures but still find the extra resolution can help show details I wouldn't see otherwise.
6MP for prints? Has been plenty for the ones I've seen from my 300D....certainly plenty for the 12x8's I put on my wall :)